Abstract:
A micropile foundation system, comprising 253 micropiles (total 12,900 ft of casing), was installed in 10 weeks with an innovative casing advancement system using two rigs (ABI TM 20/25 and FES FR-180X). The site was adjacent to New York Metropolitan Transportation Authority (MTA) tracks which increased the need to prevent ground loss. Casing was advanced through overburden soils using a recently developed system specifically designed to prevent ground loss, coupled with a pneumatically driven down-the-hole hammer (DTH). Â Depth to bedrock varied from 35 to 77 ft, casing was advanced from the ground surface to bedrock in single sections (40 to 80 ft long) without breaking rods. The DTH engaged a driver bit interlocked with a crown bit at the bottom of casing, pulling down casing (without rotation) from the bottom of drill string. This efficient procedure (without stopping to add/remove drill rods or casing sections) enabled rapid installation of micropiles using less sophisticated equipment and a less labor-intensive procedure. With casing keyed into rock, sub-casing anchor sockets were subsequently drilled in one operation using the same driver-bit after disengaging the crown bit (rotate 90-degrees counterclockwise and drop through). Anchor sockets were inspected with a down-hole camera system (owned and operated by the foundation contractor), eliminating the need for load tests. The down-hole camera system enabled direct observation of casing key-in to rock, rock condition within the anchor socket, debris within socket, and length of socket (bond zone). The paper will discuss details of the installation and challenges successfully overcome, including pneumatically advancing casing through stiff clay, large fractures within bedrock, adequate flushing for camera inspection, and meeting the production schedule.